This project will consist of a 2-year intervention development study (IDS) followed by a 3-year intervention study (IS). The overall objective of the project is to test whether wearing a weighted vest can prevent muscular weakness and osteoporosis. First, an IDS will be conducted to further refine the vest dose and to assess whether biomechanical approximations of bone loading by vest use can be measured. Second, a randomized controlled trial will be conducted to test the effect of the vest on muscular strength and bone mineral density. The nylon vest can be weighted to a maximum of 12 kg and is prescribed as a percent of body weight (%BW). Subjects wear the vest during routine daily activities (eg, chores, walks) for 2 hours per day, 4 days per week. No additional specific physical activity prescription is required. The IDS will consist of a 27-week, 3-armed (control, 3%BW, 5%BW) pre-post study in 60 subjects to compare the effects of these doses on knee extensor strength, side-effects, and adherence. Prior vest studies suggest that 5%BW is the maximum tolerated in this population; thus, maximum and 1/2 maximum dose are selected for the IS. The dose for the RCT will be decided based on the effect of each on: knee extensor strength; safety; and adherence. The IDS will evaluate whether proposed biomechanical measures of hip loading, hip joint reaction forces and hip moments-of-force, change with vest use. The effects of each vest dose on: 1) bone turnover indicators; 2) gait and balance; 3) self-reported functional status and observed physical performance will be assessed. Adherence to vest use 3 and 6 months after the termination of the intervention will also be examined. The IS will consist of a 24-month, single blinded, two armed randomized controlled trial in 66 subjects. It will examine the effects of one dose of the vest (determined by the IDS) compared to an attention control group on the following: 1) total hip BMD; 2) amount and time course of change in isokinetic knee extensor strength; and 3) long-term safety of and adherence to vest use. If the IDS has been successful in quantifying the postulated biomechanical measures of hip loading, a fourth primary objective of the RCT will be to assess the ability of the biomechanical measures to predict changes in BMD, to explore the mechanism by which the vest accomplishes bone tropism. The secondary RCT objectives include measurement of the effect of vest use on: spinal BMD; total body calcium; body composition; bone turnover markers; gait and balance; functional status; and physical performance.